Lignin-derived thermoplastic co-polymers and methods of preparation

ABSTRACT

The present invention relates to a crosslinked lignin comprising a lignin structure having methylene or ethylene linking groups therein crosslinking between phenyl ring carbon atoms, wherein said crosslinked lignin is crosslinked to an extent that it has a number-average molecular weight of at least 10,000 g/mol, is melt-processible, and has either a glass transition temperature of at least 100° C., or is substantially soluble in a polar organic solvent or aqueous alkaline solution. Thermoplastic copolymers containing the crosslinked lignin are also described. Methods for producing the crosslinked lignin and thermoplastic copolymers are also described.

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to thermoplastic biopolymers,and more particularly, to such polymers in which lignin is incorporated.

BACKGROUND OF THE INVENTION

Lignin, a complex O-methyl-substituted polyphenol found inlignocellulosic biomass, is the third most abundant natural polymer(after cellulose and chitin, respectively) that accounts for up to 30%by weight of wood. Lignin is a valuable by-product of the pulp and paperindustry produced in quantities exceeding 200 million metric tonsannually, but the vast majority of this by-product (>99%) is notisolated but combusted in the form of “black liquor” to meet the energyneeds of the mill. Future and upcoming biorefineries that processlignocellulosic materials for the production of fuel and commodityproducts (e.g., ethanol or liquid alkanes) are also expected to producelignin in large amounts as a byproduct.

Significant commercial potential exists in the conversion of lignin tohigh-value end products (i.e., functional materials), but lignin remainsa highly difficult and challenging material to convert into such usefulproducts. Although functional plastic materials have been produced fromlignin, these materials are generally highly-crosslinked thermosets,which are not melt-processible (and hence, not recyclable). Furthermore,due to their rigid and brittle character, the known lignin-containingplastics generally lack the rubber elasticity, strength, and toughnessrequired for use in many industrial and commercial applications (e.g.,automobile interior or exterior materials). Moreover, the knownlignin-containing plastics are generally not amenable for being adjustedor fine-tuned in such characteristics as glass transition temperature(T_(g)), degree of stiffness (i.e., rigidity), ductility, tensilefailure strength, and toughness, thereby significantly limiting theirrange of applications.

SUMMARY OF THE INVENTION

The instant invention is directed to high performance lignin-basedthermoplastics (thermoplastic copolymers) useful as industrial plasticresins and commercial materials for a number of applications. Thethermoplastic copolymer described herein includes crosslinked lignincopolymerized with non-lignin thermoplastic polymer segments. By theintegration of crosslinked lignin and non-lignin thermoplasticcomponents, a lignin-based thermoplastic with a combination ofadvantageous properties has herein been achieved. The resultingthermoplastic material generally exhibits a multi-phase (e.g., two-phaseor three-phase) morphology. For a special case when lignin andnon-lignin components are thermodynamically miscible, the resultingthermoplastic material generally exhibits a substantially homogeneousmorphology.

The lignin-based thermoplastics described herein are characterized bysignificant strength, durability, and ruggedness, while at the same timehaving a sufficient degree of moldability, elasticity, and/or ductilityto make them integratable into a number of different applications.Furthermore, by the methods described herein for their manufacture, awide range of different lignin-based thermoplastics differing inmechanical properties can be produced. Thus, a particularly advantageousaspect of the instant invention is the ability, by methods describeherein, to carefully adjust and fine-tune any of a variety ofcharacteristics of the thermoplastic. In particular aspects, carefulselection of the crosslinked lignin component, the non-ligninthermoplastic component, and the molar ratio used, and other variables,can be used for adjusting the characteristics of the final thermoplasticmaterial.

In preferred embodiments, the lignin-based thermoplastic is produced bya method in which a crosslinked lignin is reacted with non-ligninthermoplastic polymer segments containing lignin-reactive groupsthereon. By the method, crosslinked lignin units are covalently linkedwith non-lignin thermoplastic polymer segments.

The invention is also directed to the crosslinked lignin used forproducing the thermoplastic copolymer. The crosslinked lignin describedherein has a lignin structure in which methylene and/or ethylene linkinggroups crosslink phenyl ring carbon atoms. Significantly, by virtue ofthe mild crosslinking conditions used in processes described herein forproducing the crosslinked lignin, the crosslinked lignin describedherein is substantially less crosslinked than crosslinked lignins of theart. This substantially reduced amount of crosslinking renders thecrosslinked lignin melt-processible, even when the crosslinked ligninpossesses a number-average or weight-average molecular weight of atleast 10,000 g/mol or a significantly higher weight. Moreover, themelt-processible crosslinked lignin described herein is generallycharacterized by a glass transition temperature of at least 100° C.,and/or substantial or complete solubility in a polar organic solvent oraqueous alkaline solution. Solvation of crosslinked lignin into thenon-lignin component of the copolymer significantly enhancesmelt-processibility of the product.

In preferred embodiments, the crosslinked lignin is produced by a methodin which a precursor lignin having a number-average molecular weight ofup to or less than 10,000 g/mol is reacted with an aqueous solutioncontaining up to or less than 10 weight percent formaldehyde and/orglyoxal under condensation conditions. The resulting crosslinked ligninincludes methylene and/or ethylene linking groups crosslinking betweenphenyl ring carbon atoms, and possesses the physical characteristics andproperties described above.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Some possible structures of lignin-based multiphase copolymers.

FIG. 2. Differential scanning calorimetry (DSC) traces of as-receivedlignin and modified, or pre-crosslinked lignin at three differentconditions, showing a significant increase in T_(g).

FIGS. 3A, 3B. Viscosity (left, A) and shear modulus (right, B) oflignin-rubber copolymers and controls.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention is directed to a crosslinked ligninthat is melt-processible or amenable to melt-processing. By being“crosslinked” is meant that the lignin contains methylene (i.e., —CH₂—)and/or ethylene (i.e., —CH₂CH₂—) linkages (i.e., linking groups) betweenphenyl ring carbon atoms in the lignin structure. By being“melt-processible” is meant that the crosslinked lignin can be melted orconverted to a molten, highly viscous, or rubbery state starting at aparticular glass transition temperature. The melted or highly viscouslignin can then be more easily processed, such as by mixing, molding,applying on a surface, or dissolving in a solvent.

The crosslinked lignin is crosslinked to an extent that it has anumber-average or weight-average molecular weight (i.e., M_(n) or M_(w),respectively) of at least 10,000 g/mol. In different embodiments, thecrosslinked lignin has a number-average or weight-average molecularweight of precisely, about, at least, or greater than, for example,10,000 g/mol, 25,000 g/mol, 50,000 g/mol, 75,000 g/mol, 100,000 g/mol,125,000 g/mol, 150,000 g/mol, 175,000 g/mol, or 200,000 g/mol, or amolecular weight within a range bounded by any two of the foregoingexemplary values.

The glass transition temperature (T_(g)) of the crosslinked lignin isgenerally above room temperature (typically, 15, 20, 25, or 30° C.). Indifferent embodiments, the crosslinked lignin has a glass transitiontemperature of precisely, about, at least, or greater than 40° C., 50°C., 60° C., 70° C., 80° C., 90° C., 100° C., 105° C., 110° C., 115° C.,120° C., 125° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C.,190° C., 200° C., 210° C., 220° C., 230° C., 240° C., or 250° C., or aT_(g) within a range bounded by any two of the foregoing values.

As used herein, the term “about” generally indicates within ±0.5%, 1%,2%, 5%, or up to ±10% of the indicated value. For example, a molecularweight of about 10,000 g/mol generally indicates, in its broadest sense,10,000 g/mol±10%, which indicates 9,000-11,000 g/mol. In addition, theterm “about” can indicate either a measurement error (i.e., bylimitations in the measurement method), or alternatively, a variation oraverage in a physical characteristic of a group (e.g., a variation inmolecular weights).

The crosslinked lignin is preferably substantially soluble in a polarorganic solvent or aqueous alkaline solution. As used herein, the term“substantially soluble” generally indicates that at least 1, 2, 5, 10,20, 30, 40, or 50 grams of the crosslinked lignin completely dissolvesin 1 deciliter (100 mL) of the polar organic solvent or aqueous alkalinesolution. In other embodiments, the solubility is expressed as a wt % ofthe crosslinked lignin in solution. In particular embodiments, thecrosslinked lignin has sufficient solubility to produce at least a 5 wt%, 10 wt %, 15 wt %, 20 wt %, 30 wt %, 40 wt %, or 50 wt % solution inthe polar organic solvent or aqueous alkaline solution. The polarorganic solvent can be aprotic or protic. Some examples of polar aproticsolvents include the organoethers (e.g., diethyl ether, tetrahydrofuran,and dioxane), nitriles (e.g., acetonitrile, propionitrile), sulfoxides(e.g., dimethylsulfoxide), amides (e.g., dimethylformamide,N,N-dimethylacetamide), organochlorides (e.g., methylene chloride,chloroform, 1,1,-trichloroethane), ketones (e.g., acetone, 2-butanone),and dialkylcarbonates (e.g., ethylene carbonate, dimethylcarbonate,diethylcarbonate). Some examples of polar organic protic solventsinclude the alcohols (e.g., methanol, ethanol, isopropanol, n-butanol,t-butanol, the pentanols, hexanols, octanols, or the like), diols (e.g.,ethylene glycol, diethylene glycol, triethylene glycol), and proticamines (e.g., ethylenediamine, ethanolamine, diethanolamine, andtriethanolamine). The aqueous alkaline solution can be anyaqueous-containing solution having a pH of at least (or over) 8, 9, 10,11, 12, or 13. The alkalizing solute can be, for example, an alkalihydroxide (e.g., NaOH or KOH), ammonia, or ammonium hydroxide.Combinations of any of these solvents may also be used. In someembodiments, one or more classes or specific types of solvents areexcluded.

The invention is also directed to a composition in which the crosslinkedlignin is dissolved in one or a combination of an organic solvent oraqueous alkaline solution. The resulting composition is a solution ofthe crosslinked lignin in a suitable solvent or solvent mixture.Additional ingredients may also be included, such as to preventdegradation or bacterial growth during storage or use.

In another aspect, the invention is directed to a method for producingthe crosslinked lignin described above. In the method, a precursorlignin is treated, under condensation conditions, with an aqueoussolution containing formaldehyde and/or glyoxal at a substantially lowerconcentration than conventionally practiced in the art. In differentembodiments, the low concentration of formaldehyde and/or glyoxal usedin the reaction is precisely, about, up to, or less than, for example,0.5 wt %, 1 wt %, 1.5 wt %, 2 wt %, 2.5 wt %, 3 wt %, 3.5 wt %, 4 wt %,4.5 wt %, 5 wt %, 5.5 wt %, 6 wt %, 6.5 wt %, 7 wt %, 7.5 wt %, 8 wt %,8.5 wt %, 9 wt %, 9.5 wt %, or 10 wt % relative to total volume orweight of the reaction contents, or a concentration within a rangebounded by any two of these exemplary values. Such mild concentrationsof formaldehyde and/or glyoxal can be achieved by appropriate dilutionof an initial solution of formaldehyde and/or glyoxal of anyconcentration. Typically, the initial solution of formaldehyde and/orglyoxal before dilution is no more than 5 wt %, 10 wt %, 20 wt %, or 30wt %. The term “condensation conditions” refer to those conditions,generally known in the art, that cause aldehydic and phenolic groups tocrosslink. Generally, an elevated temperature in the presence of a baseor acid catalyst is used for sufficient time to cause substantialcrosslinking. In different embodiments, the elevated temperature can be,for example, about, at least, or above 50° C., 60° C., 70° C., 80° C.,90° C., or 100° C., sustained for a period of, for example, 30, 45, 60,90, 120, 150, 180, 210, 240, 270, 300, 330, 360, 390, or 420 minutes,wherein it is understood that, generally, higher temperatures requireless time to achieve the same or similar amount of crosslinking.

In the method for producing the crosslinked lignin, thealdehyde-containing species (typically, formaldehyde and/or glyoxal) ispreferably in a mole ratio to lignin phenolic groups (i.e., F/P ratio)of at least 1:1 and up to 1:200. In different embodiments, the F/P ratiois precisely, about, or at least, for example, 1:1, 1:2, 1:3, 1:4, 1:5,1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80, 1:90, 1:100, 1:120,1:150, or 1:200, or a F/P ratio within a range bounded by any two of theforegoing values.

The precursor lignin preferably has a number-average or weight-averagemolecular weight of up to or less than 10,000 g/mol. In differentembodiments, the precursor lignin has a molecular weight of up to orless than, for example, 10,000 g/mol, 9,500 g/mol, 9,000 g/mol, 8,500g/mol, 8,000 g/mol, 7,500 g/mol, 7,000 g/mol, 6,500 g/mol, 6,000 g/mol,5,500 g/mol, 5,000 g/mol, 4,500 g/mol, 4,000 g/mol, 3,500 g/mol, 3,000g/mol, 2,500 g/mol, 2,000 g/mol, 1,500 g/mol, 1,250 g/mol, or 1,000g/mol, or a molecular weight within a range bounded by any two of theforegoing exemplary values. In further embodiments, the precursorlignin, or lignol monomer can have a molecular weight of at least 100g/mol, 150 g/mol, 180 g/mol, 200 g/mol, 210 g/mol, 250 g/mol, 300 g/mol,500 g/mol, or 750 g/mol, and an upper molecular weight corresponding toany of the exemplary maximum molecular weights provided above (e.g.,500-3000 g/mol or 500-2000 g/mol).

The precursor lignin can be any of a wide variety of lignin compositionsfound in nature or as known in the art. As known in the art, there is nouniform lignin composition found in nature. Lignin is a random polymerthat shows significant compositional variation between plant species.Many other conditions, such as environmental conditions, age, and methodof processing, influence the lignin composition. Lignins differ mainlyin the ratio of three alcohol units, i.e., p-coumaryl alcohol, guaiacylalcohol, and sinapyl alcohol. The polymerization of p-coumaryl alcohol,coniferyl alcohol, and sinapyl alcohol forms the p-hydroxyphenyl (H),guaiacyl (G) and syringyl (S) components of the lignin polymer,respectively. The precursor lignin can have any of a wide variety ofrelative weight percents (wt %) of H, G, and S components. For example,the precursor lignin may contain, independently for each component, atleast, up to, or less than 1 wt %, 2 wt %, 5 wt %, 10 wt %, 20 wt %, 30wt %, 40 wt %, 50 wt %, 60 wt %, 70 wt %, 80 wt %, or 90 wt %, or arange thereof, of any of the H, G, and S components. Typically, the sumof the wt % of each H, G, and S component is 100%, or at least 98% ifother minor components are considered. Different wood and plant sources(e.g., hardwood, softwood, switchgrass, and bagasse) often widely differin their lignin compositions.

Besides the natural variation of lignins, there can be furthercompositional variation based on the manner in which the lignin has beenprocessed. For example, the precursor lignin can be a Kraft lignin,sulfite lignin (i.e., lignosulfonate), or a sulfur-free lignin. As knownin the art, a Kraft lignin refers to lignin that results from the Kraftprocess. In the Kraft process, a combination of sodium hydroxide andsodium sulfide (known as “white liquor”) is reacted with lignin to forma dark-colored lignin bearing thiol groups. Kraft lignins are generallywater- and solvent-insoluble materials with a high concentration ofphenolic groups. They can typically be made soluble in aqueous alkalinesolution. As also known in the art, sulfite lignin refers to lignin thatresults from the sulfite process. In the sulfite process, sulfite orbisulfite (depending on pH), along with a counterion, is reacted withlignin to form a lignin bearing sulfonate (SO₃H) groups. The sulfonategroups impart a substantial degree of water-solubility to the sulfitelignin. There are several types of sulfur-free lignins known in the art,including lignin obtained from biomass conversion technologies (such asthose used in ethanol production), solvent pulping (i.e., the“organosolv” process), and soda pulping. In particular, organosolvlignins are obtained by solvent extraction from a lignocellulosicsource, such as chipped wood, followed by precipitation. Due to thesignificantly milder conditions employed in producing organosolv lignins(i.e., in contrast to Kraft and sulfite processes), organosolv ligninsare generally more pure, less degraded, and generally possess a narrowermolecular weight distribution than Kraft and sulfite lignins. Any one ormore of the foregoing types of lignins may be used (or excluded) as aprecursor lignin in the method described herein for producing acrosslinked lignin.

The lignin may also be an engineered form of lignin having a specific oroptimized ratio of H, G, and S components. Lignin can be engineered by,for example, transgenic and recombinant DNA methods known in the artthat cause a variation in the chemical structure in lignin and overalllignin content in biomass (e.g., F. Chen, et al., Nature Biotechnology,25(7), pp. 759-761 (2007) and A. M. Anterola, et al., Phytochemistry,61, pp. 221-294 (2002)). The engineering of lignin is particularlydirected to altering the ratio of G and S components of lignin (D. Guo,et al., The Plant Cell, 13, pp. 73-88, (January 2001). In particular,wood pulping kinetic studies show that an increase in S/G ratiosignificantly enhances the rate of lignin removal (L. Li, et al.,Proceedings of The National Academy of Sciences of The United States ofAmerica, 100 (8), pp. 4939-4944 (2003)). The S units become covalentlyconnected with two lignol monomers; on the other hand, G units canconnect to three other units. Thus, an increased G content (decreasingS/G ratio) generally produces a highly branched lignin structure withmore C—C bonding. In contrast, increased S content generally results inmore β-aryl ether (β-O-4) linkages, which easily cleave (as compared toC—C bond) during chemical delignification, e.g., as in the Kraft pulpingprocess. It has been shown that decreasing lignin content and alteringthe S/G ratio improve bioconvertability and delignification. Thus, lessharsh and damaging conditions can be used for delignification (i.e., ascompared to current practice using strong acid or base), which wouldprovide a more improved lignin better suited for higher value-addedapplications, including carbon fiber production and pyrolytic orcatalytic production of aromatic hydrocarbon feedstock.

Lab-scale biomass fermentations that leave a high lignin content residuehave been investigated (S. D. Brown, et al., Applied Biochemistry andBiotechnology, 137, pp. 663-674 (2007)). These residues will containlignin with varied molecular structure depending on the biomass source(e.g., wood species, grass, and straw). Production of value-addedproducts from these high quality lignins would greatly improve theoverall operating costs of a biorefinery. Various chemical routes havebeen proposed to obtain value-added products from lignin (J. E.Holladay, et al., Top Value-Added Chemicals from Biomass: VolumeII—Results of Screening for Potential Candidates from BiorefineryLignin, DOE Report, PNNL-16983 (October 2007)).

In some embodiments, an additional phenolic species (e.g., phenol,resorcinol, or the like), is excluded from the method of producing thecrosslinked lignin. In some embodiments, an aldehyde-containing species(e.g., an aldehyde or dialdehyde) other than formaldehyde and glyoxal isexcluded from the method of producing the crosslinked lignin. Someexamples of such aldehydes or dialdehydes that may be excluded includemalondialdehyde, succindialdehyde, glutaraldehyde, and acetaldehyde. Insome embodiments, the method includes solely the precursor lignin andeither formaldehyde or glyoxal, or a combination thereof, in a suitablesolvent, optionally including one or more auxiliary agents necessary ordesired for facilitating the reaction (e.g., a catalytic species, pHadjuster, buffer, surfactant, and the like).

In another aspect, the invention is directed to a thermoplasticcopolymer that contains the crosslinked lignin described above. Thethermoplastic copolymer contains units (i.e., segments) of theabove-described crosslinked lignin covalently linked (i.e.,copolymerized) with one or more types of thermoplastic polymer unitsthat are not lignin (i.e., non-lignin thermoplastic polymer segments).The resulting copolymer can include terpolymer and tetrapolymercompositions. The copolymer can be, for example, a block, graft,alternating, or random copolymer, or a combination thereof. Depending onthe chemistry employed, the crosslinked lignin segments can function asharder or more rigid segments compared to the non-lignin polymersegments, or the crosslinked lignin segments can function as softer ormore elastic segments compared to the non-lignin polymer segments. Thecopolymer is generally characterized by having a two-phase (or higherphase) morphology. Some possible arrangements for the lignin-basedmultiphase copolymers are shown in FIG. 1.

The non-lignin thermoplastic polymer segments can have any thermoplasticpolymer composition known in the art. In some embodiments, thenon-lignin thermoplastic polymer segments are saturated (i.e., do notcontain carbon-carbon double or triple bonds). In other embodiments, thenon-lignin thermoplastic polymer segments are unsaturated by havingcarbon-carbon double or triple bonds. Natural and synthetic rubbers areparticular examples of thermoplastic polymers containing carbon-carbondouble bonds. The non-lignin thermoplastic polymer segments may,themselves, be linear or branched polymer segments, or copolymericsegments containing thermoplastic sub-segments therein, wherein thecopolymeric segments can be, for example, a block, graft, alternating,or random copolymer, or a combination thereof. In some embodiments, thenon-lignin thermoplastic polymer segments are copolymers containinglignin sub-segments (which may be compositionally the same or differentfrom the crosslinked lignin component). In other embodiments, thenon-lignin thermoplastic polymer segments do not contain ligninsub-segments.

In different embodiments, the non-lignin thermoplastic polymer segmentscan have any of a wide range of glass transition temperatures (T_(g)),such as a T_(g) of precisely, about, at least, above, up to, or lessthan, for example, −100° C., −50° C., 0° C., 10° C., 20° C., 30° C., 40°C., 50° C., 60° C., 70° C., 80° C., 90° C., 100° C., 110° C., 120° C.,130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., or 200°C., or a T_(g) within a range bounded by any two of the foregoingexemplary values. The thermoplastic copolymer containing the non-ligninthermoplastic polymer segments and crosslinked lignin segments may alsohave a T_(g) selected from any of the exemplary values provided above orwithin a range bounded by any two of the above exemplary values.

In different embodiments, the non-lignin thermoplastic polymer segmentscan have any of a wide range of weight-average molecular weights(M_(w)), such as precisely, about, at least, above, up to, or less than,for example, 500,000 g/mol, 400,000 g/mol, 300,000 g/mol, 200,000 g/mol,100,000 g/mol, 50,000 g/mol, 10,000 g/mol, 5,000 g/mol, 2,500 g/mol,2,000 g/mol, 1,500 g/mol, 1,000 g/mol, 500 g/mol, or 250 g/mol. Thenon-lignin thermoplastic polymer segments may also have any of a widerange of number-average molecular weights wherein n can correspond toany of the numbers provided above for M_(w), as well as, for example, 3,4, 5, 10, 20, 50, 100, or 200, and wherein the may correspond to any ofthe M_(w) values provided above. The molecular weight of the non-ligninthermoplastic polymer segments can also be within a range bounded by anytwo of the foregoing exemplary values. The thermoplastic copolymercontaining the non-lignin thermoplastic polymer segments and crosslinkedlignin segments may also have a M_(w) selected from any of the exemplaryvalues provided above, including M_(w) values larger than 500,000 g/mol,or within a range bounded by any two of the foregoing exemplary values.

In particular embodiments, the non-lignin thermoplastic polymer segmentsare derived from monomer units having a chemical structure within thefollowing generic chemical structure:

In Formula (1), each of R¹, R², R³, R⁴, R⁵, and R⁶ can independently bea hydrogen atom, a saturated or unsaturated hydrocarbon group having 1to 4 carbon atoms, or a halogen atom. Some examples of saturatedhydrocarbon groups having 1 to 4 carbon atoms include methyl, ethyl,n-propyl, isopropyl, n-butyl, sec-butyl, t-butyl, cyclopropyl, andcyclobutyl. Some examples of unsaturated hydrocarbon groups having 1 to4 carbon atoms include vinyl (—CH═CH₂), allyl (—CH₂—CH═CH₂), 1-propenyl(—CH═CH—CH₃), 3-butenyl (—CH₂—CH₂—CH═CH₂), 1,3-butadienyl(—CH═CH—CH═CH₂), ethynyl, and 2-propynyl. Halogen atoms include, forexample, fluoro, chloro, and bromo atoms.

By being independently selected, R¹, R², R³, R⁴, R⁵, and R⁶ can beindependently selected within a monomer unit as well as between monomerunits. In some embodiments, a polymer derived from Formula (1) containsmonomer units that are unvarying between monomer units with respect toR¹, R², R³, R⁴, R⁵, and R⁶ (i.e., each monomer unit contains the sameR¹, R², R³, R⁴, R⁵, and R⁶ groups, although each R¹, R², R³, R⁴, R⁵, andR⁶ is independently selected within a monomer unit, and thus, may or maynot be the same within a monomer unit). In other embodiments, a polymerderived from Formula (1) contains monomer units that vary betweenmonomer units with respect to R¹, R², R³, R⁴, R⁵, and R⁶ (i.e., thepolymer contains one or more different types of monomer units, therebybeing a copolymer). In some embodiments, a copolymer may be derived frommonomer precursors of Formula (1) in combination with at least one othertype of monomer unit, such as ethylene, a fluoroethylene, an acrylateacid or ester, a methacrylate acid or ester, acrylonitrile, styrene,divinylstyrene, or a combination thereof (e.g., wherein the copolymer isABS).

In one embodiment, R¹, R², R³, R⁴, R⁵, and R⁶ are all hydrogen atoms,which results in Formula (1) being butadiene, and hence, the non-ligninthermoplastic polymer segments being polybutadiene or a copolymerderived from butadiene and one or more other monomer units, such asethylene, an acrylate, a methacrylate, a fluoro or chloro ethylene,acrylonitrile, or isoprene. In other embodiments, at least one, two,three, four, five, or all of R¹, R², R³, R⁴, R⁵, and R⁶ are selectedfrom hydrocarbon groups (e.g., methyl groups), or halogen atoms, or amixture thereof. In particular embodiments, one or both of R³ and R⁴ aremethyl groups, while R¹, R², R⁵, and R⁶ are hydrogen atoms. In the casewhere one of R³ and R⁴ is a methyl group, Formula (1) corresponds toisoprene, and hence, the non-lignin thermoplastic polymer segmentscorrespond to polyisoprene or a copolymer derived from isoprene and oneor more other monomer units, such as ethylene, an acrylate, amethacrylate, a fluoro or chloro ethylene, acrylonitrile, or butadiene.

In other particular embodiments, the non-lignin thermoplastic polymersegments are, or include, alkyleneoxide (polyoxyalkylene) polymer units.The alkyleneoxide polymer units can be any of the alkyleneoxide polymersin the art, such as ethyleneoxide, propylene oxide, or butylene oxidepolymers. The alkyleneoxide polymer units may also be a copolymer of,for example, ethyleneoxide and propylene oxide (typically a diblock ortriblock copolymer), or a copolymer of an alkyleneoxide and acarboxyl-containing monomer, such as a polyoxyalkylene ester. Thecopolymer may also be modified by inclusion of a branching moiety, suchas ethylene glycol or glycerol molecules, or aliphatic or aromaticdicarboxylic or tricarboxylic molecules. Numerous alkyleneoxide polymersand copolymers are known in the art, all of which are considered herein,unless they significantly undermine desired characteristics of the finalthermoplastic polymer, such as the toughness and elasticity requirementsfor a particular application.

In yet other particular embodiments, the non-lignin thermoplasticpolymer segments are, or include, a polymeric structure with a saturatedbackbone within the following generic chemical structure:

In Formula (2), R⁷, R⁸, R⁹, and R¹⁰ are independently selected fromhydrogen atom, saturated or unsaturated hydrocarbon groups having 1 to 4carbon atoms (as defined above for R¹, R², R³, R⁴, R⁵, and R⁶), nitrile,halogen atoms, and groups having formulas —C(O)R¹¹, C(O)OR¹², and —OR¹³,wherein R¹¹, R¹², and R¹³ are selected from hydrogen atom and saturatedor unsaturated hydrocarbon groups having 1 to 4 carbon atoms (as definedabove). The subscript n can be, for example, at least 2, 3, 4, 5, 10,20, 30, 40, 50, 100, 200, 300, 400, 500, 1000, 5000, or a value within arange therein. Formula (2) can represent a monomer or copolymer. In someembodiments, R⁷, R⁸, R⁹, and R¹⁰ are all hydrogen atoms for at least aportion of monomer units. In other embodiments, at least one or two ofR⁷, R⁸, R⁹, and R¹⁰ (for at least a portion of monomer units) areselected from saturated or unsaturated hydrocarbon groups having 1 to 4carbon atoms, and/or nitrile, and/or halogen atom, and/or a group of theformula —C(O)R¹¹, and/or a group of the formula C(O)OR¹², and/or a groupof the formula —OR¹³. In the case where R⁷, R⁸, R⁹ and R¹⁰ are allhydrogen, the polymer segment corresponds to polyethylene. In the casewhere R⁷, R⁸ and R⁹ are all hydrogen and R¹⁰ is a methyl group, thepolymer segment corresponds to polypropylene. In some embodiments, whenR⁷ and R⁸ are hydrogen atoms, and R⁹ and R¹⁰ are methyl groups, thepolymer segment corresponds to polyisobutylene. In the case where R⁷, R⁸and R⁹ are all hydrogen and R¹⁰ is a vinyl group, the polymer segmentcorresponds to poly(1,2-butadiene). In the case where R⁷, R⁸ and R⁹ areall hydrogen and R¹⁰ is an ethyl group, the polymer segment correspondsto polybutylene. In the case where R⁷, R⁸ and R⁹ are all hydrogen andR¹⁰ is a phenyl group, the polymer segment corresponds to polystyrene.

In particular embodiments the soft non-lignin polymer segment is acopolymer of a polymer segment derived from Formula (1) and a polymersegment of Formula (2). For example, when Formula (1) is butadiene (toproduce a polybutadiene segment) and Formula (2) is polystyrene, thecopolymer segment is styrene-butadiene rubber (SBR). Similarly, inanother embodiment, the non-lignin segment is a copolymer of twodifferent polymers of Formula (2), such as a copolymer of ethylene andbutylene (i.e., ethylene-butylene copolymer). In yet another embodiment,the soft-segment is a terpolymer from Formula (2), such as a terpolymerof styrene, ethylene, and butylene (i.e., styrene-ethylene-butyleneterpolymer).

In still other particular embodiments, the non-lignin thermoplasticpolymer segments are, or include, a hydroxy acid structure (e.g., α- or(β-hydroxy acid structure), and more particularly, apoly(hydroxyalkanoate) (i.e., “polyhydroxyalkanoate”) structure. Inparticular embodiments, the polyhydroxyalkanoate structure is defined bythe following generic chemical structure:

In Formula (3), R¹⁴ is selected from a hydrogen atom (H) or hydrocarbongroup. The hydrocarbon group for R¹⁴ includes any of the saturated orunsaturated hydrocarbon groups described above as having one to fourcarbon atoms, as well as hydrocarbon groups having higher than fourcarbon atoms, such as five or six carbon atoms. Some examples ofhydrocarbon groups having five or six carbon atoms include n-pentyl,isopentyl, cyclopentyl, n-hexyl, isohexyl, cyclohexyl, and phenylgroups. The subscript t is typically an integer from 0 to 3 (i.e., t istypically 0, 1, 2, or 3). The subscript n is typically an integer of atleast 5, 10, 20, 50, or 100, 200, 500, 1000, 1500, 2000, 2500, or 5000,or within a range therein. The structure shown by Formula (3) can be amonomer or copolymer. When t is 0, Formula (3) depicts a polymer of analpha-hydroxy (α-hydroxy) acid. An example of an α-hydroxy polymer whenR¹⁴ is H is polyglycolic acid. An example of an α-hydroxy polymer whenR¹⁴ is methyl is polylactic acid (which may also be poly-L-lactic acid,poly-D-lactic acid, or poly-DL-lactic acid). An example of an cc-hydroxypolymer when R¹⁴ is phenyl is polymandelic acid. When t is 1, Formula(3) depicts a polymer of a beta-hydroxy (β-hydroxy) acid. An example ofa β-hydroxy acid corresponding to R¹⁴ being H is 3-hydroxypropionicacid. An example of a β-hydroxy acid corresponding to R¹⁴ being methylis β-hydroxybutyric acid, the monomeric unit of polyhydroxybutyrate(i.e., PHB or P3HB). The hydroxy acid need not be within the scope ofFormula (3) to be suitable. For example, a polymer of salicylic acid mayalso be a suitable hydroxy acid polymer.

Copolymers of the hydroxy acids are also considered herein. In someembodiments, two or more different types of hydroxyalkanoates are in thecopolymer, such as in (poly(lactic-co-glycolic acid). In otherembodiments, the copolymer includes one or more non-hydroxyalkanoateportions, as in poly(glycolide-co-caprolactone) andpoly(glycolide-co-trimethylene carbonate).

In different embodiments, the thermoplastic copolymer containing thenon-lignin thermoplastic polymer segments and crosslinked ligninsegments preferably exhibits an angular shear rate viscosity of at least500 Pa·s, 600 Pa·s, 700 Pa·s, 800 Pa·s, 900 Pa·s, 1000 Pa·s, 1200 Pa·s,1500 Pa·s, 5000 Pa·s, 10,000 Pa·s, 25,000 Pa·s, 50,000 Pa·s, 75,000Pa·s, 100,000 Pa·s, 200,000 Pa·s, 300,000 Pa·s, 400,000 Pa·s, or 500,000Pa·s at an angular frequency of up to 1000 rad/s at room temperature, oran angular shear rate viscosity within a range bounded by any two of theforegoing exemplary values.

In different embodiments, the thermoplastic copolymer containing thenon-lignin thermoplastic polymer segments and crosslinked ligninsegments preferably exhibits a shear modulus of at least 1 Pa, 5 Pa, 10Pa, 50 Pa, 100 Pa, 200 Pa, 300 Pa, 400 Pa, 500 Pa, 600 Pa, 700 Pa, 800Pa, 900 Pa, 1000 Pa, 1200 Pa, 1500 Pa, 5000 Pa, 10,000 Pa, 50,000 Pa, or100,000 Pa at an angular frequency of up to 10 rad/s (typically at roomtemperature), or a shear modulus within a range bounded by any two ofthe foregoing exemplary values.

In different embodiments, the thermoplastic copolymer containing thenon-lignin thermoplastic polymer segments and crosslinked ligninsegments preferably exhibits a strength of at least 1 MPa, 5 MPa, 10MPa, 15 MPa, 20 MPa, 25 MPa, 30 MPa, 40 MPa, 50 MPa, or 100 MPa, or astrength within a range bounded by any two of the foregoing exemplaryvalues.

In different embodiments, the thermoplastic copolymer containing thenon-lignin thermoplastic polymer segments and crosslinked ligninsegments preferably exhibits an elongation of at least 1%, 2%, 3%, 4%,5%, 6%, 7%, 8%, 9%, 10%, 12%, 15%, 20%, 50%, or 100%, or an elongationwithin a range bounded by any two of the foregoing exemplary values.

The amount (i.e., weight percent, or “wt %”) of crosslinked lignin inthe thermoplastic copolymer can be any suitable amount that produces adesirable set of characteristics in the thermoplastic copolymer. Indifferent embodiments, the crosslinked lignin can be in an amount ofprecisely, about, at least, up to, or less than, for example, 1 wt %, 2wt %, 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40wt %, 45 wt %, 50 wt %, 55 wt %, 60 wt %, 65 wt %, 70 wt %, 75 wt %, 80wt %, 85 wt %, 90 wt %, 95 wt %, 98 wt %, or 99 wt % by weight of thethermoplastic copolymer. The amount of crosslinked lignin in thethermoplastic copolymer may also be within a range bounded by any two ofthe exemplary values provided above (for example, 10-60 wt %, 15-60 wt%, 20-60 wt %, 10-50 wt %, 15-50 wt %, or 20-50 wt %).

In another aspect, the invention is directed to a process for preparingthe thermoplastic copolymer described above. In the method, acrosslinked lignin, as described above, is reacted with non-ligninthermoplastic polymer segments, as described above, but modified tocontain lignin-reactive groups thereon. Generally, at least twolignin-reactive groups are present per each non-lignin thermoplasticpolymer segment. The lignin-reactive groups present on the modifiednon-lignin thermoplastic polymer segments permit the non-ligninthermoplastic polymer segments to covalently bind with the crosslinkedlignin. The lignin-reactive groups can be any groups reactive to one ormore types of groups that may be present on the crosslinked lignin. Thereactive groups can be reactive to, for example, phenol hydroxy groups(as commonly found in lignin), alcoholic groups in general, furanylgroups, or any other groups on the lignin, such as those that may beincluded by chemical modification. Some examples of lignin-reactivegroups include carboxylic acid, carboxylic acid ester, acyl chloride,epoxy, and isocyanate groups. Alternatively, the crosslinked lignin maybe suitably derivatized to include groups (e.g., any of the reactivegroups described above, including amino or mercapto groups) reactive togroups on a modified or unmodified form of the non-lignin thermoplasticpolymer segments.

The chemical modification of non-lignin thermoplastic polymers toinclude such lignin-reactive groups is well known in the art, and manysuch derivatized polymers are commercially available (e.g.,alkyleneoxide polymers derivatized to contain at least twophenol-reactive groups, such as epoxy, aldehyde, or carboxylic acidester groups). Notably, some of the non-lignin thermoplastic polymersdescribed herein naturally contain lignin-reactive groups withoutfurther modification, e.g., the hydroxy acids, such as thepolyhydroxyalkanoates. In particular embodiments, the modifiednon-lignin thermoplastic polymer is a polybutadiene-containing polymeror copolymer derivatized with carboxy or epoxy groups, particularlythose end-capped with carboxy groups, such as a carboxylated orepoxidized polybutadiene polymer of the following structures:

The conditions used for linking the non-lignin thermoplastic polymersegments with crosslinked lignin generally include any of the conditionsknown in the art for facilitating a reaction between any of the commonpairs of reactive groups described above. Some of the conditionsgenerally well known in the art include, for example, production of anester from carboxylic acid and alcohol groups, or a carbamate group fromisocyanate and alcohol groups. In some of the reactions, a catalyticspecies (such as an acid or base) and/or an elevated temperature areused to facilitate the reaction. In some embodiments, the crosslinkedlignin and non-lignin thermoplastic polymer segments are reacted underin situ melt mixing conditions to enhance mechanical properties. Inother embodiments, the crosslinked lignin and non-lignin thermoplasticpolymer segments are reacted under partial free-radical crosslinkingconditions. The free radicals are typically generated by decompositionof organic peroxides, such as benzoyl peroxide and dicumyl peroxide. Theconditions employed in these and other polymerization techniques arewell known in the art. For example, in a melt-mixing process in aninternal mixer SBR is pre-mixed with low dosage (0.5 g per hundred grubber) of organic peroxide, such as dicumyl peroxide at about 60° C. Ina second step, the lignin is charged into an empty internal mixer at180° C. and sheared for 1 minute, and then peroxide pre-mixed SBR isloaded at 50-50 lignin-SBR ratio. When melt-mixed mass was thermallymolded for two different compositions, one with peroxide and the otherwithout peroxide, peroxide crosslinked specimens showed 50% highertensile strength. Depending on the processing conditions, respectiveperoxide crosslinked formulations showed 0.5 MPa to 5 MPa tensilestrength.

In some embodiments, the lignin crosslinking and copolymerizationprocesses are conducted as separate processes. In other embodiments, thelignin crosslinking and copolymerization processes are conducted as asingle (i.e., combined, or “one-pot”) process, i.e., in a singlereaction vessel.

In some embodiments, an additional phenolic species (e.g., phenol,resorcinol, or the like), is excluded from the method of producing thethermoplastic copolymer. In some embodiments, the method includes solelythe crosslinked lignin and non-lignin thermoplastic polymer asreactants, typically in a suitable solvent, optionally including one ormore auxiliary agents necessary or desired for facilitating the reaction(e.g., a catalytic species, pH adjuster, buffer, surfactant, and thelike).

In particular embodiments, lignin segments are copolymerized withpolyester molecules to enhance mechanical properties.Transesterification in the presence of an acid catalyst leads to theinsertion of more rigid lignin segments into bio-polyester segments,resulting in an increased T_(g) of the new bio-derived materials. Anexemplary transesterification process is shown schematically for thetrans-esterification of PHB at primary and secondary alcohol sites oflignin, as follows:

In other particular embodiments, crosslinked lignin is copolymerizedwith epoxy-functionalized polybutadiene. An exemplary process forepoxidizing polybutadiene and copolymerizing crosslinked lignin withepoxy-functionalized polybutadiene is shown in the following reactionscheme:

In other particular embodiments lignin segments are free-radical graftedonto natural rubber (e.g., latex of the Hevea tree). A low dosage ofperoxide initiator, such as benzoyl or dicumyl peroxide, is generallyused to produce free radical sites on natural rubber (polyisoprene). Theability of lignin to act as an antioxidant (hindered phenol type)generally permits the efficient coupling of lignin to these free radicalsites, typically producing comb-like structures of grafted lignin-latexchains. This route advantageously minimizes the influence of chaintermination by lignin impurities (e.g., carbohydrates), allowingsynthesis of high molecular weight polymers without intensive ligninpurification procedures.

In other aspects, the invention is directed to a process in which thelignin crosslinking process described above is integrated with alignin-producing process. The process that produces lignin can be, forexample, a pulp or paper manufacturing process, or a biorefineryprocess.

Typically, biorefineries produce ethanol (as a product) and lignin (as abyproduct) from a lignocellulosic biomass source. Some examples ofsuitable lignocellulosic biomass materials include wood, corn stover,Populus (e.g., poplar, aspen, and/or cottonwood), switchgrass (i.e.,Panicum virgatum), miscanthus, sugarcane, paper pulp, and hemp. In abiorefinery, biomass is generally initially pre-treated by boiling,steaming, and/or with dilute acid to loosen cellulose, hemicellulose,and other carbohydrate components in biomass from lignin. Thispretreatment process is generally followed by saccharification (i.e.,production of sugar, such as glucose, by use of a cellulase enzyme oncellulose), and then fermentation of the sugar by enzymes and/or anethanologen microbe (e.g., yeast) to produce ethanol. Due to therelatively mild chemical process generally employed in abiomass-to-ethanol biorefinery, the lignin residue emanating from thebiorefinery is generally less degraded and more conserved from itsnatural state, and generally has a higher molecular weight, than thatisolated from conventional pulp processing operations, such as the Kraftpulping industry. By being “integrated”, the equipment used incrosslinking the lignin (by methods described above) is typicallycontained within, physically integrated with, and/or physicallyconnected with the equipment used in the lignin-producing operation. Forexample, the lignin-producing operation may include a ligninprecipitation or extraction vessel, from which lignin, after beingprecipitated or extracted (and optionally, further purified) istransferred by suitable mechanical means to a lignin crosslinkingstation.

In still other aspects, the invention is directed to an articlecontaining the crosslinked lignin or thermoplastic copolymer describedabove. The article is typically one in which a significant degree oftoughness is desired along with a degree of elasticity. In particularembodiments, the article is in the interior of an automobile (e.g., seatand interior covering), the surface of a piece of furniture, a grip orhandle portion of a tool or utensil, or a mat. In other embodiments, thecrosslinked lignin or thermoplastic copolymer may be produced andapplied as a coating or film, such as a protective film. Numerous otherarticles may make use of the compositions described herein. Thecrosslinked lignin may also be incorporated in a formulation as, forexample, a thickening, binding, coating, adhesive, or dispersing agent.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

EXAMPLE 1 Controlled Crosslinking of Lignin

The process for crosslinking lignin described in this example can begenerally summarized by the following reaction scheme:

A 1-L round-bottomed flask was equipped with a large magnetic stir barand placed in a heating mantle. 100 g of organosolv lignin (as received,not dried, having a MW of 1840 g/mol, polydispersity index (ratio ofweight average to number average molecular weights) of 122, and T_(g) of108° C.) were added to the flask. Lignin was dissolved using 200 mL NaOHsolution (pH>14) and 300 mL of millipure water, for a finalconcentration of 16 wt % solids. Once the lignin appeared fullydissolved, the flask was fitted with an addition funnel. 120 mL offormalin solution (10 wt % in buffered solution) were added dropwise tothe lignin solution with stirring, at 80° C. After two hours, anadditional 120 mL of formalin (10 wt % in buffer solution) were added tothe reaction. Reaction continued for four additional hours (six hourstotal).

The reaction flask was removed from the heating mantle and allowed tocool. The lignin product was precipitated by the bubbling of carbondioxide through the solution, and isolated by decanting the solvent. Thesolid product was dried at 40° C. to yield 70 g of material. The drylignin powder exhibited a MW of 31,000 g/mol, polydispersity index of20.7, T_(g)>200° C., and complete solubility in N,N-dimethylformamide(DMF). Thus, this pre-crosslinking method not only enhances themolecular weight, but also lowers the polydispersity index withoutsignificantly affecting its solubility.

Through careful tuning of reaction conditions, higher molecular weightlignin fractions were achieved through crosslinking while maintainingsolubility in DMF, tetrahydrofuran, and alkaline solution. A significantincrease in glass transition temperature (T_(g)) from 107° C. to greaterthan 160° C. occurred due to changes in lignin structure (FIG. 2).Lignin-1, Lignin-2, and Lignin-3 are the compositions made with 1:4,1:2, 1:1, formaldehyde to lignin phenolic molar ratios, respectively.With increase in degree of crosslinking introduced by the methylenebridging groups, the lignin polymer's segmental mobility issignificantly reduced. This, in turn, enhances the glass transitiontemperature, or softening point.

EXAMPLE 2 Copolymerization Reactions of Crosslinked Lignin

I. Copolymerization Reaction of Crosslinked Lignin withCarboxy-Functionalized Polybutadiene

The process for copolymerizing crosslinked lignin withcarboxy-functionalized polybutadiene described in this example can begenerally summarized by the following reaction scheme. The long curvedlines shown in the final product represent carboxy-functionalizedpolybutadiene that has been attached to the crosslinked lignin.

As-received lignin was reacted with dicarboxy-terminated polybutadienesoft segment (M_(n) 4,200 g/mol) in the presence of formalin at 1:1 F/Lratio with KOH in 1,4-dioxane solvent (with KOH concentration of 2.4mmol/40 mL solvent) for 24 hours at 100° C. Different amounts of ligninwere loaded to obtain 9, 13, 17, and 22 wt % lignin, respectively, inthe feed compositions. All the resulting lignin-polybutadiene copolymerswere isolated via precipitation into methanol and dried under reducedpressure at 50° C. for 18 hours.

II. Results

In each case, the synthesized product was a thick, sticky brown solid, ahybrid of the dark brown powder lignin and clear viscous liquid rubber.The dynamic shear viscosity and shear modulus of the lignin-rubbercopolymer increased by two orders of magnitude over the neat polymersoft segment or a blend of both components, demonstrating that chemicalbonding did occur (FIG. 3). Furthermore, an increase in the amount oflignin incorporated corresponded with an increase in modulus. At thehighest lignin content obtained, shear thinning behavior was observed inthe synthesized copolymer. It was observed that covalent bond formationbetween lignin and telechelic polybutadiene caused significantentanglement in the resulting polymer. The entanglement, polymer chainrigidity, and the viscosity increased with increase in lignin content inthe copolymer formulations. As shown in FIG. 3(A), the control physicalmixture (not processed under reaction conditions) at 17 wt % lignincontent showed four times higher viscosity than the liquid rubber. Thisslight increase in viscosity is due to the incorporation of powderedlignin into the liquid rubber. However, the reacted composition of thesame lignin content shows 40 times higher viscosity than the neatrubber, a result that evidences network formation by bonding betweenlignin and rubber. The formulation with slightly higher lignin content(22 wt %) showed a shear thinning behavior at low angular frequency, aresult that indicates that the physical network due to entanglement ofthe polymers is disrupted by an increase in angular frequency. However,the plateau viscosities at very high angular frequencies are relativelyclose. The corresponding storage shear moduli of the compositions aredisplayed in FIG. 3(B). With an increase in frequency, the rubberymaterials exhibit strain hardening and higher modulus. However, as thelignin content increases, the modulus also increases. The non-reactedblend does not show significantly enhanced modulus compared to thecopolymerized composition.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A crosslinked lignin comprising a ligninstructure having methylene or ethylene linking groups thereincrosslinking between phenyl ring carbon atoms, wherein said crosslinkedlignin is crosslinked to an extent that it has a number-averagemolecular weight of at least 10,000 g/mol, is melt-processible, and haseither a glass transition temperature of at least 100° C., or issubstantially soluble in a polar organic solvent or aqueous alkalinesolution.
 2. The crosslinked lignin of claim 1, wherein saidnumber-average molecular weight is at least 100,000 g/mol.
 3. Thecrosslinked lignin polymer of claim 1, wherein said number-averagemolecular weight is at least 150,000 g/mol.
 4. The crosslinked ligninpolymer of claim 1, wherein said number-average molecular weight is atleast 200,000 g/mol.
 5. The crosslinked lignin polymer of claim 1,wherein said glass transition temperature is at least 120° C.
 6. Thecrosslinked lignin polymer of claim 1, wherein said glass transitiontemperature is at least 150° C.
 7. The crosslinked lignin polymer ofclaim 1, wherein said glass transition temperature is at least 180° C.8. The crosslinked lignin polymer of claim 1, wherein said glasstransition temperature is at least 200° C.
 9. A thermoplastic copolymer,wherein said thermoplastic copolymer has a two-phase morphology and iscomprised of crosslinked lignin copolymerized with non-ligninthermoplastic polymer segments, wherein said crosslinked lignin iscomprised of a lignin structure having methylene or ethylene linkinggroups therein crosslinking between phenyl ring carbon atoms, and saidcrosslinked lignin is crosslinked to an extent that it has anumber-average molecular weight of at least 10,000 g/mol, ismelt-processible, and has a glass transition temperature of at least100° C., or is substantially soluble in a polar organic solvent oraqueous alkaline solution.
 10. The thermoplastic copolymer of claim 9,wherein said thermoplastic copolymer is a block copolymer or multiphasecopolymer.
 11. The thermoplastic copolymer of claim 9, wherein saidthermoplastic copolymer is a graft copolymer.
 12. The thermoplasticcopolymer of claim 9, wherein said thermoplastic copolymer has a glasstransition temperature selected from a temperature in the range of −100°C. up to 200° C.
 13. The thermoplastic copolymer of claim 9, whereinsaid non-lignin thermoplastic polymer segments contain unsaturatedcarbon-carbon bonds.
 14. The thermoplastic copolymer of claim 13,wherein said non-lignin thermoplastic polymer segments are derived frommonomer units having a chemical structure within the following genericchemical structure:

wherein R¹, R², R³, R⁴, R⁵, and R⁶ are independently selected fromhydrogen atom, a saturated or unsaturated hydrocarbon group having 1 to4 carbon atoms, and halogen atoms.
 15. The thermoplastic copolymer ofclaim 14, wherein said non-lignin thermoplastic polymer segmentscomprise polyisoprene units.
 16. The thermoplastic copolymer of claim14, wherein said non-lignin thermoplastic polymer segments comprisepolybutadiene units.
 17. The thermoplastic copolymer of claim 9, whereinsaid non-lignin thermoplastic polymer segments are alkyleneoxide polymerunits.
 18. The thermoplastic copolymer of claim 17, wherein saidalkylene-oxide polymer units are ethyleneoxide polymer units.
 19. Thethermoplastic copolymer of claim 9, wherein said non-ligninthermoplastic polymer segments possess a saturated backbone and have achemical structure within the following generic chemical structure:

wherein R⁷, R⁸, R⁹, and R¹⁰ are independently selected from hydrogenatom, saturated or unsaturated hydrocarbon groups having 1 to 4 carbonatoms, nitrile, halogen atoms, and groups having formulas —C(O)R¹¹,C(O)OR¹², and —OR¹³, wherein R11, R^(12,) and R¹³ are selected fromhydrogen atom and saturated or unsaturated hydrocarbon groups having 1to 4 carbon atoms, and n is an integer of at least 2, and said genericchemical structure can be a monomer or copolymer.
 20. The thermoplasticcopolymer of claim 9, wherein said non-lignin thermoplastic polymersegments are comprised of a polyhydroxyalkanoate structure within thefollowing generic chemical structure:

wherein R¹⁴ is selected from a hydrogen atom or hydrocarbon group, t isan integer from 0 to 3, n is an integer of at least 5, and said genericstructure can be a monomer or copolymer.
 21. The thermoplastic copolymerof claim 9, wherein said thermoplastic copolymer exhibits an angularshear rate viscosity of at least 500 Pa·s at an angular frequency of upto 1000 rad/s at room temperature.
 22. The thermoplastic copolymer ofclaim 9, wherein said thermoplastic copolymer exhibits an angular shearrate viscosity of at least 1000 Pa·s at an angular frequency of up to1000 rad/s at room temperature.
 23. The thermoplastic copolymer of claim9, wherein said thermoplastic copolymer exhibits a shear modulus of atleast 100 Pa at an angular frequency of up to 10 rad/s.
 24. Thethermoplastic copolymer of claim 9, wherein said thermoplastic copolymerexhibits a shear modulus of at least 1000 Pa at an angular frequency ofup to 10 rad/s.
 25. The thermoplastic copolymer of claim 9, wherein saidthermoplastic copolymer exhibits a shear modulus of at least 1200 Pa atan angular frequency of up to 10 rad/s.
 26. The thermoplastic copolymerof claim 9, wherein said thermoplastic copolymer contains at least 10weight percent and up to 60 weight percent of said crosslinked lignin.27. The thermoplastic copolymer of claim 26, wherein said thermoplasticcopolymer contains at least 15 weight percent and up to 50 weightpercent of said crosslinked lignin.
 28. The thermoplastic copolymer ofclaim 26, wherein said thermoplastic copolymer contains at least 20weight percent and up to 50 weight percent of said crosslinked lignin.29. A method for preparing a crosslinked lignin, the process comprisingtreating a precursor lignin having a number-average molecular weight ofup to 10,000 g/mol with formaldehyde and/or glyoxal, present in aconcentration of up to 10 wt % of reaction volume, under condensationconditions to produce said crosslinked lignin, wherein said crosslinkedlignin includes methylene and/or ethylene linking groups thereincrosslinking between phenyl ring carbon atoms, wherein said crosslinkedlignin is crosslinked to an extent that it has a number-averagemolecular weight of at least or greater than 10,000 g/mol, ismelt-processible, and has either a glass transition temperature of atleast 100° C., or is substantially soluble in a polar organic solvent oraqueous alkaline solution.
 30. The method of claim 29, wherein saidformaldehyde and/or glyoxal is present in a concentration of up to 5 wt% of reaction volume.
 31. The method of claim 29, wherein saidformaldehyde and/or glyoxal is in a mole ratio to lignin phenolic groupsof 1:10 to 1:100.
 32. The method of claim 29, wherein said precursorlignin has a number-average molecular weight of at least 500 g/mol. 33.The method of claim 29, wherein said precursor lignin has anumber-average molecular weight of up to 3,000 g/mol.
 34. A method forpreparing a thermoplastic copolymer, the method comprising reacting acrosslinked lignin with non-lignin thermoplastic polymer segmentscontaining lignin-reactive groups thereon, wherein said crosslinkedlignin is comprised of a lignin structure having methylene or ethylenelinking groups therein crosslinking between phenyl ring carbon atoms,and said crosslinked lignin is crosslinked to an extent that it has anumber-average molecular weight of at least 10,000 g/mol, ismelt-processible, and has either a glass transition temperature of atleast 100° C., or is substantially soluble in a polar organic solvent oraqueous alkaline solution.
 35. The method of claim 34, wherein saidlignin-reactive groups are selected from carboxylic acid, carboxylicacid ester, acyl chloride, epoxy, and isocyanate groups.
 36. The methodof claim 34, wherein said non-lignin thermoplastic polymer segments arederived from monomer units having a chemical structure within thefollowing generic chemical structure:

wherein R¹, R², R³, R⁴, R⁵, and R⁶ are independently selected fromhydrogen atom, a saturated or unsaturated hydrocarbon group having 1 to4 carbon atoms, and halogen atoms, and wherein each of said non-ligninthermoplastic polymer segments according to formula (1) includes atleast two lignin-reactive groups.
 37. The method of claim 36, whereinsaid non-lignin thermoplastic polymer segments are comprised ofpolyisoprene units.
 38. The method of claim 36, wherein said non-ligninthermoplastic polymer segments are comprised of polybutadiene units. 39.The method of claim 34, wherein said non-lignin thermoplastic polymersegments are alkyleneoxide polymer units, each containing at least twophenol-reactive groups.
 40. The method of claim 34, wherein saidnon-lignin thermoplastic polymer segments possess a saturated backboneand have a chemical structure within the following generic chemicalstructure:

wherein R⁷, R⁸, R⁹, and R¹⁰ are independently selected from hydrogenatom, saturated or unsaturated hydrocarbon groups having 1 to 4 carbonatoms, nitrile, halogen atoms, and groups having formulas —C(O)R¹¹,C(O)OR¹², and —OR¹³, wherein R11, R^(12,) and R¹³ are selected fromhydrogen atom and saturated or unsaturated hydrocarbon groups having 1to 4 carbon atoms, and n is an integer of at least 2, wherein saidgeneric chemical structure can be a monomer or copolymer and includes atleast two lignin-reactive groups.
 41. The method of claim 34, whereinsaid non-lignin thermoplastic polymer segments are comprised ofpolyhydroxyalkanoate structure within the following generic chemicalstructure:

wherein R¹⁴ is selected from a hydrogen atom or hydrocarbon group, t isan integer from 0 to 3, n is an integer of at least 5, and said genericstructure can be a monomer or copolymer and includes at least twolignin-reactive groups.
 42. The method of claim 34, wherein saidthermoplastic copolymer exhibits an angular shear rate viscosity of atleast 500 Pa·s at an angular frequency of up to 1000 rad/s at roomtemperature.
 43. The method of claim 34, wherein said thermoplasticcopolymer exhibits a shear modulus of at least 100 Pa at an angularfrequency of up to 10 rad/s.
 44. The method of claim 34, wherein saidthermoplastic copolymer contains at least 10 weight percent and up to 50weight percent of said crosslinked lignin.
 45. The method of claim 34,wherein said crosslinked lignin and non-lignin thermoplastic polymersegments are reacted under in situ melt mixing polymerizationconditions.
 46. The method of claim 34, wherein said crosslinked ligninand non-lignin thermoplastic polymer segments are reacted underfree-radical grafting polymerization conditions.